Reversal of the adaptive response of neuropeptide Y neurons in the rat striatum to nigrostriatal dopamine deafferentation by the N-methyl-D-aspartate antagonist dizocilpine maleate

Increased GABAergic transmission in neuropeptide Y-expressing neurons in the dopamine-depleted murine striatum

As the main input nucleus of the basal ganglia, the striatum plays a central role in planning, control, and execution of movement and motor skill learning. More than 90% of striatal neurons, so-called medium spiny neurons (MSN), are GABAergic projection neurons, innervating primarily the substantia nigra pars reticulata or the globus pallidus internus. The remaining neurons are GABAergic and cholinergic interneurons, synchronizing and controlling striatal output by reciprocal connections with MSN. Besides prominent local cholinergic influence, striatal function is globally regulated by dopamine (DA) from the nigrostriatal pathway. Little is known about whether DA depletion, as occurs in Parkinson's disease, affects the activity of striatal interneurons. Here we focused on neuropeptide Y (NPY)-expressing interneurons, which are among the major subgroups of GABAergic interneurons in the striatum. We investigated the effects of striatal DA depletion on GABAergic transmission in NPY interneurons by electrophysiologically recording GABAergic spontaneous (s) and miniature (m) inhibitory postsynaptic currents (IPSCs) in identified NPY interneurons in slices from 6-hydroxydopamine (6-OHDA)- and vehicle-injected transgenic NPY-humanized Renilla green fluorescent protein (hrGFP) mice with the whole cell patch-clamp technique. We report a significant increase in sIPSC and mIPSC frequency as well as the occurrence of giant synaptic and burst sIPSCs in the 6-OHDA group, suggesting changes in GABAergic circuit activity and synaptic transmission. IPSC kinetics remained unchanged, pointing to mainly presynaptic changes in GABAergic transmission. These results show that chronic DA depletion following 6-OHDA injection causes activity-dependent and -independent increase of synaptic GABAergic inhibition onto striatal NPY interneurons, confirming their involvement in the functional impairments of the DA-depleted striatum.NEW & NOTEWORTHY Neuropeptide Y (NPY) interneurons regulate the function of striatal projection neurons and are upregulated upon dopamine depletion in the striatum. Here we investigated how dopamine depletion affects NPY circuits and show electrophysiologically that it leads to the occurrence of giant synaptic and burst GABAergic spontaneous inhibitory postsynaptic currents (IPSCs) and to an activity-independent increase in GABAergic miniature IPSC frequency in NPY neurons. We suggest that degeneration of dopaminergic terminals in the striatum causes functional changes in striatal GABAergic function.

Nerve growth factor retrieves neuropeptide Y and cholinergic immunoreactivity in the nucleus accumbens of old rats

The nucleus accumbens (NAc) contains high levels of neuropeptide Y (NPY), which is involved in the regulation of functions and behaviors that deteriorate with aging. We sought to determine if aging alters NPY expression in this nucleus and, in the affirmative, if those changes are attributable to the cholinergic innervation of the NAc. The total number and the somatic volume of NPY- and choline acetyltransferase-immunoreactive neurons, and the density of cholinergic varicosities were estimated in the NAc of adult (6 months old) and aged (24 months old) rats. In aged rats, the number of NPY neurons was reduced by 20% and their size was unaltered. The number of cholinergic neurons and the density of the cholinergic varicosities were unchanged, but their somas were hypertrophied. Nerve growth factor administration to aged rats further increased the volume of cholinergic neurons, augmented the density of the cholinergic varicosities, and reversed the age-related decrease in the number of NPY neurons. Our data show that the age-related changes in NPY levels in the NAc cannot be solely ascribed to the cholinergic innervation of the nucleus.

Alterations in striatal neuropeptide Y system activity of rats with haloperidol-induced behavioral supersensitivity

The study was conducted to determine whether the expression of behavioral supersensitivity induced by haloperidol (HAL) administered once daily (2 mg/kg i.p.) for 14 days is associated with the alterations in activity of neuropeptide Y (NPY) system in the striatum (caudate-putamen) and nucleus accumbens. Dopamine supersensitivity was tested by measurement of locomotor activity and stereotyped behavior after administration of the dopamine D2/D3 receptor agonist quinpirole (1 mg/kg i.p.) on day 1, 3 and 7 after HAL withdrawal. Neuropeptide Y-like immunoreactivity (NPY-LI) was determined in the striatum and nucleus accumbens isolated 6 h after quinpirole injection on day 1, 3 and 7 after the end of HAL treatment. NPY mRNA was quantified in these structures on day 7 after HAL withdrawal. HAL increased spontaneous locomotor activity and prevalence of rearing, grooming and head-down sniffing. At the same time, striatal NPY-LI increased progressively from the reduced level found on day 1 of haloperidol withdrawal. NPY mRNA remained unchanged. In saline-treated rats, quinpirole enhanced locomotion, rearing, and induced intense head-down sniffing and oral activity. These behavioral effects were accompanied by a decrease in striatal NPY-LI. NPY mRNA was slightly increased. HAL treatment altered response to quinpirole, namely it increased locomotion, intensified oral activity and reduced rearing and head-down sniffing. The second and the third quinpirole injection decreased NPY-LI levels. NPY mRNA was unchanged. In the nucleus accumbens, apart from a decrease in NPY-LI on day 1 after the last haloperidol dose, the level of NPY-LI and NPY mRNA in any experimental group did not differ from the control value. The presented results suggest that the alterations in the activity of the striatal but not nucleus accumbens NPY system contribute to adaptive changes induced by long-term haloperidol treatment and may be of significance to the motor hyperactivity induced by intermittent stimulation of postsynaptic dopamine D2 receptors.

Down-regulation of nitrergic transmission in the rat striatum after chronic nigrostriatal deafferentation

Dopamine and NO are physiological stimulators of synthesis of cAMP and cGMP, respectively, and NO synthase-containing interneurons in the striatum are physiologically activated by dopamine-containing neurons in the substantia nigra. This study investigated whether lesioning dopamine neurons has multiple consequences in the striatum consistent with the reported sensitization of cAMP synthesis, including alteration of the NO-cGMP pathway and phosphodiesterase-dependent metabolism of cyclic nucleotides. The substantia nigra of adult Sprague-Dawley rats was unilaterally lesioned with 6-hydroxydopamine. Two months later, we determined expression of NO synthase and evaluated cGMP and cAMP levels of intact and deafferented striatum. Moreover, we evaluated cAMP- and cGMP-phosphodiesterase activities in basal conditions and after Ca2+-calmodulin stimulation and determined the expression of the phosphodiesterase-1B isoform and the levels of phosphodiesterase-1B mRNA. Using immunocytochemistry we characterized the distribution of NO synthase and phosphodiesterase-1B within striatal neurons. In the dopamine-deafferented striatum, NO synthase levels were decreased by 42% while NO synthase-immunopositive intrastriatal fibres but not NO synthase neuronal bodies were reduced in number. In the deafferented striatum basal cGMP levels were reduced, and cAMP levels were increased, but cGMP-phosphodiesterase and cAMP-phosphodiesterase activities were both increased in basal and Ca2+-calmodulin-stimulated conditions. Accordingly, phosphodiesterase-1B expression and phosphodiesterase-1B mRNA were upregulated while a large population of medium-sized striatal neurons showed increased phosphodiesterase-1B immunoreactivity. Dopamine deafferentation led to a complex down-regulation of the NO-cGMP pathway in the striatum and to an up-regulation of phosphodiesterase-1B-dependent cyclic nucleotide metabolism, showing new aspects of neuronal plasticity in experimental hemiparkinsonism.

Increased neuropeptide Y mRNA expression in striatum in Parkinson's disease

High levels of neuropeptide Y (NPY) are found in basal ganglia where it is co-localised with somatostatin (SOM) and nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH/d) in a population of striatal GABA containing interneurones. Although alterations occur in the levels of various neuropeptides in basal ganglia in Parkinson's disease (PD), it is not known whether NPY is affected. Using in situ hybridisation immunohistochemistry, we have examined the distribution of NPY mRNA in the caudate nucleus, putamen and nucleus accumbens of normal individuals and patients with PD. NPY mRNA was weakly expressed in the caudate nucleus, putamen and nucleus accumbens in normal individuals with a scattered labelling of neurones. However, there was no regional localisation within any brain area and no obvious differences between brain regions. In PD, the number of NPY mRNA-expressing cells was increased as was the density of the silver grains overlying each positive cell. The increase was more pronounced in the nucleus accumbens and in the ventral part of the caudate nucleus. The increase in NPY mRNA expression observed in patients with PD may reflect the loss of dopaminergic tone on striatal NPY containing interneurones, although a role for chronic L-DOPA therapy cannot be ruled out.

Effects of haloperidol and clozapine on neuropeptide Y-like immunoreactivity in the nucleus accumbens and striatum of rats pretreated with psychostimulants

It has been shown that the synthesis, release and levels of neuropeptide Y (NPY), a peptide linked to the dopamine system, are altered by stimulants with psychotomimetic properties and by antipsychotic drugs. This study was designed to evaluate the effect of 3-day haloperidol (HAL) (2 mg/kg i.p.) or clozapine (CLOZ) (25 mg/kg i.p.) treatment on neuropeptide Y-like immunoreactivity (NPY-LI) in nucleus accumbens and striatum (caudate-putamen) in rats pretreated with d-amphetamine or phencyclidine. D-amphetamine (5 mg/kg s.c. twice daily for 6 days and once on day 7) and phencyclidine (10 mg/kg i.p. once daily for 2 days and 15 mg/kg once on day 3) induced marked stereotypy with different symptomatology. Stereotypy is thought to resemble psychosis-related behavior. The first dose of either HAL or CLOZ was given 3 or 2h after the final d-amphetamine or phencyclidine injection, respectively. The control groups were injected with either saline alone, saline instead of psychostimulants, which was followed by antipsychotics, or psychostimulants followed by saline. Rats were sacrificed 24h after antipsychotics or 72 h after the last psychostimulant dose. Both psychostimulants similarly reduced nucleus accumbens and striatal NPY-LI. In saline-pretreated rats, HAL and CLOZ decreased nucleus accumbens NPY-LI, but only HAL decreased striatal NPY-LI. In both the structures examined the effects of HAL and d-amphetamine on NPY-LI were additive. HAL slightly enhanced but CLOZ reversed the phencyclidine-induced decrease in accumbens NPY-LI. The present study has shown that HAL and CLOZ produce different effects on nucleus accumbens and striatal NPY-LI in d-amphetamine or phencyclidine pretreated rats, and it suggests that these effects are related particularly to the dopaminergic and glutamatergic systems whose activities were altered by psychostimulants.

The effect of competitive and non-competitive NMDA receptor antagonists, ACPC and MK-801 on NPY and CRF-like immunoreactivity in the rat brain amygdala

Amygdala is the brain structure responsible for integrating all behavior connected with fear, and in this structure two neuropeptides, neuropeptide Y (NPY), corticoliberin (CRF) and the most abundant excitatory neurotransmitter glutamate seem to take part in the regulation of anxiety behavior. Our previous studies showed the modulation of NPY and CRF expression by classical neurotransmitters in some brain structures, therefore in the present study we investigated the effect of NMDA receptor antagonists on the expression of NPY and CRF immunoreactivity in the rat brain amygdala. A non-competitive NMDA receptor antagonist, MK-801, or a functional NMDA antagonist, ACPC were used. Brains were taken out and processed by immunohistochemical method using specific NPY or CRF antibodies. The staining intensity and density of IR neurons were evaluated under a microscope in amygdala sections. It was found that both MK-801 and ACPC induced a significant decrease in NPY-immunoreactivity in amygdala nerve cell bodies and terminals, which may suggest an increased release of this peptide. CRF-IR was decreased after ACPC only. The obtained results indicate that in the amygdala, the NMDA receptors mediated glutamatergic transmission may regulate NPY neurons.

Effects of typical and atypical antipsychotics on neuropeptide Y in rat brain tissue and microdialysates from ventral striatum

The main goal of this study was to investigate effects of typical (haloperidol) and atypical (risperidone) antipsychotic drugs on brain regional neuropeptide Y (NPY)-like immunoreactivity (-LI) tissue concentrations and on release of NPY-LI in freely moving rats. An additional aim was to explore the effect of d-amphetamine on NPY-LI release following pretreatment with typical and atypical antipsychotics. During a 4-week period, male Wistar rats were fed chow to which vehicle, risperidone (1.15 mg/100 g food or 2.3 mg/100 g food), or haloperidol (1.15 mg/100 g food) were added. In one series of experiments, the animals were sacrificed on day 30 with focused microwave irradiation, the brain regions dissected and extracted for radioimmunoassay of NPY-LI. In another experimental series, probes were inserted into the ventral striatum. The perfusates were collected at 60-min intervals; NPY-LI was determined by radioimmunoassay. Haloperidol significantly increased NPY-LI in hypothalamus and the occipital cortex. In contrast, haloperidol decreased tissue levels of NPY-LI in striatum. Moreover, haloperidol and risperidone also significantly decreased extracellular NPY-LI concentrations in the ventral striatum. d-amphetamine (1.5 mg/kg) significantly increased extracellular NPY-LI in the vehicle group. Both haloperidol and risperidone pretreatments abolished the effect of d-amphetamine. The results show that d-amphetamine as well as haloperidol and risperidone selectively and specifically affect NPY-LI concentrations in brain tissue and microdialysates and that the effect of d-amphetamine is abolished by both typical and atypical antipsychotics.

Ultrastructural and metabolic changes in the neuropeptide Y-containing striatal neuronal network after thermocoagulatory cortical lesion in adult rat

This study examined the effects of unilateral thermocoagulatory cortical lesion on the pattern of neuropeptide Y immunostaining in the rat ipsilateral striatum at 4 and 21 days post-lesion. Light microscopic analysis showed a significant increase in the number of neuropeptide Y-positive neurons vs. control at both time points; paradoxically, the intraneuronal level of labelling significantly decreased at 4 days post-lesion but increased at 21 days post-lesion. Ultrastructural analysis in control condition showed a higher proportion of dendritic versus axonal labelled processes (3.5 ratio); all the neuropeptide Y synaptic terminals formed symmetrical contacts, mostly onto unlabelled dendrites. At 4 days post-lesion, the neuropeptide Y-positive axon density dramatically increased (+576%) without significant change in the labelled dendrite density, vs. control values; the density of neuropeptide Y synaptic terminals increased in parallel by 233%. In addition, a significant proportion of large neuropeptide Y boutons forming asymmetrical synapses onto unlabelled spines were observed. At 21 days post-lesion, densities of neuropeptide Y dendrites, axons, and synaptic terminals increased by 68, 246 and 125%, respectively, vs. control. But, the morphological features of the neuropeptide Y axonal processes and synaptic specializations of the boutons were similar to those observed in control condition. These data (1) raise an important issue regarding the origin of the terminals forming asymmetrical synapses in the striatum, (2) suggest that adaptative changes in the neuropeptide Y neuronal network may be a main component of striatal remodelling resulting from the progressive loss of cortical inputs, and (3) reinforce the view that neuropeptide Y and excitatory amino acid functions may be tightly linked in the striatum.

Differential modulation of NMDA-stimulated [3H]dopamine release from rat striatum by neuropeptide Y and sigma receptor ligands

Although the identity of the endogenous ligands for sigma (sigma) receptors is unknown, neuropeptide Y (NPY) has been named as a possible candidate for a natural transmitter at these receptors. Using a superfusion system, we compared the effect of NPY on NMDA-stimulated [3H]dopamine release in rat striatum to that of the sigma agonists (+)-pentazocine and BD737. In contrast to (+)-pentazocine- or BD737-mediated inhibition of release, NPY enhanced release. However, the same sigma antagonists (BD1008, DuP734, haloperidol and DTG) that reverse (+)-pentazocine- or BD737-mediated inhibition, as well as a Y receptor antagonist, PYX-1, all reversed the enhancement. PYX-1 also reversed the (+)-pentazocine- and BD737-mediated inhibition of release. Peptide YY (PYY) and [Leu31,Pro34]NPY did not mimic the effect of NPY. NPY13-36 enhanced release to the same extent as NPY but the effect was not reversed by sigma antagonists. Our findings are consistent with the potential role of NPY as an endogenous ligand for a subtype of sigma receptor with characteristics different from Y1, Y2 and Y3 receptors but sensitive to PYX-1.

Repeated injections of dizocilpine maleate (MK-801) do not suppress the effects of nigrostriatal dopamine deafferentation on glutamate decarboxylase (GAD67) mRNA expression in the adult rat striatum

The present study examined the effects of glutamate transmission blockade through N-methyl-D-aspartate (NMDA) receptor subtype by repeated administration of dizocilpine maleate (0.2 mg/kg. i.p., twice a day for eight days) alone or in combination with unilateral 6-hydroxydopamine-induced lesion of the nigrostriatal dopaminergic pathway on GABAergic neurons in the adult rat striatum. For this purpose, the expression of the messenger RNA encoding for the 67 kDa isoform of the GABA synthesizing enzyme, glutamate decarboxylase (GAD67 mRNA), was studied in the various conditions by quantitative in situ hybridization. The dizocilpine maleate treatment alone did not induce significant change of GAD67 mRNA levels in the striatum, indicating that NMDA receptors may not have a major role in the transcriptional regulation of GAD67 in the adult rat striatum. As reported previously, the unilateral dopaminergic lesion resulted in marked increases in GAD67 mRNA levels in the ipsilateral striatum. This up-regulation was not significantly affected by the treatment with dizocilpine maleate started 12 days after the unilateral intranigral 6-hydroxydopamine injection. Therefore, NMDA receptors are unlikely to contribute to the dopamine lesion-induced GAD67 mRNA up-regulation in striatal projection neurons. This result is of major interest in comparison with our previous finding that NMDA receptor activation is necessary to maintain the up-regulation of enkephalin expression in the striatum after dopamine lesion.

The contralateral cortex contributes to the effects of hemidecortication on neuropeptide Y immunoreactivity in the rat striatum

We have previously shown that unilateral lesion by thermocoagulation of sensori-motor cortex which provides excitatory afferents to the striatum increases the number of neuropeptide Y (NPY)-immunoreactive neurons in the rat striatum. The present study examined whether this paradoxical effect is due to adaptive neuronal mechanisms involving the crossed projections from the contralateral spared cortex. To test this hypothesis, we compared the effects of unilateral and bilateral cortical lesions on the number of NPY-immunoreactive neurons in the striatum. Results showed that animals with bilateral lesion have no significant change in NPY immunoreactivity versus control suggesting that the contralateral intact cortex is responsible for the increase of NPY-immunoreactive neurons detected after unilateral lesion.

Unilateral 6-hydroxydopamine lesions of meso-striatal dopamine neurons and their physiological sequelae

One of the primary approaches in experimental brain research is to investigate the effects of specific destruction of its parts. Here, several neurotoxins are available which can be used to eliminate neurons of a certain neurochemical type or family. With respect to the study of dopamine neurons in the brain, especially within the basal ganglia, the neurotoxin 6-hydroxydopamine (6-OHDA) provides an important tool. The most common version of lesion induced with this toxin is the unilateral lesion placed in the area of mesencephalic dopamine somata or their ascending fibers, which leads to a lateralized loss of striatal dopamine. This approach has contributed to neuroscientific knowledge at the basic and clinical levels, since it has been used to clarify the neuroanatomy, neurochemistry, and electrophysiology of mesencephalic dopamine neurons and their relationships with the basal ganglia. Furthermore, unilateral 6-OHDA lesions have been used to investigate the role of these dopamine neurons with respect to behavior, and to examine the brain's capacity to recover from or compensate for specific neurochemical depletions. Finally, in clinically-oriented research, the lesion has been used to model aspects of Parkinson's disease, a human neurodegenerative disease which is neuronally characterized by a severe loss of the meso-striatal dopamine neurons. In the present review, which is the first of two, the lesion's effects on physiological parameters are being dealt with, including histological manifestations, effects on dopaminergic measures, other neurotransmitters (e.g. GABA, acetylcholine, glutamate), neuromodulators (e.g. neuropeptides, neurotrophins), electrophysiological activity, and measures of energy consumption. The findings are being discussed especially in relation to time after lesion and in relation to lesion severeness, that is, the differential role of total versus partial depletions of dopamine and the possible mechanisms of compensation. Finally, the advantages and possible drawbacks of such a lateralized lesion model are discussed.

Chronic dizocilpine maleate (MK-801) treatment suppresses the effects of nigrostriatal dopamine deafferentation on enkephalin but not on substance P expression in the rat striatum

The present study examined the effects of chronic treatment with dizocilpine maleate (0.2 mg/kg i.p., twice a day for 8 days) alone or in combination with unilateral 6-hydroxydopamine-induced lesion of the nigrostriatal dopaminergic neurons on substance P and enkephalin expression in the rat striatum. This was done by means of quantitative in situ hybridization histochemistry and immunocytochemistry. As reported previously, the unilateral dopaminergic lesion resulted in marked decreases in substance P mRNA expression and immunoreactivity in the ipsilateral striatum while enkephalin mRNA expression and Met-enkephalin immunoreactivity were considerably increased in this structure. Blockade of NMDA receptors by chronic dizocilpine maleate treatment alone resulted in decreased levels of striatal substance P mRNA without significant change in substance P immunoreactivity versus controls. Enkephalin mRNA levels were also decreased in the striatum, matched by parallel reductions in Met-enkephalin immunoreactivity. These observations indicate that NMDA receptor activity may exert tonic excitatory effects on substance P and enkephalin expression in the striatum. The same chronic treatment with dizocilpine maleate started 12 days after the 6-hydroxydopamine injection suppressed the lesion-induced up-regulation of enkephalin expression without significantly affecting the down-regulation of substance P expression. These data provide evidence that NMDA receptor-mediated mechanisms contribute to the alteration of striatal enkephalin expression associated with dopaminergic depletion in hemiparkinsonian rat models.